Gear sets or gear trains are common to mechanical and electro-mechanical systems requiring rotational motion and power transmission, and are therefore often utilized in systems ranging in complexity from a simple wrist watch or wind-up toy to advanced modern automotive transmissions. A gear set consists of two or more mechanical gear elements. Each gear element is engageable, meshable, or otherwise matable with at least one other gear element in the gear set for the purpose of transmitting power and motion between the various gear elements comprising the gear set. The specific gear element or series of elements chosen for any given application will largely depend upon the dynamics of the system into which the gear set is employed, as well as the respective forces or loads to which the individual gear elements that comprise the gear set are subjected.
Complex mechanical systems, for example automotive transmissions, commonly use a planetary gear set or sets comprised of any number of inter-meshed external gear elements such as sun gears or ring gears, and internal gear elements such as pinion gears, with the terms “external” and “internal” referring to the projecting direction of the gear teeth ringing the gear element. Each mating gear element within a planetary gear set of a transmission has a plurality of mating or meshing gear teeth, with each gear tooth typically having an involute surface profile. In an involute profile, contact between mating gear teeth is retained within a flat plane as the curved flanks of the gear teeth rotatably engage and disengage, thereby isolating all physical contact between the mating gear teeth to the active or contact surface portions of the gear flanks. Positioned between the mating gear teeth are non-contactable root portions each having a generally semi-circular profile. The semi-circular profiles of the root portions of involute gear teeth are, as part of the gear formation step, typically formed by milling or hobbing processes which cut or remove excess material from the metal gear blanks. While the involute design has many known inherent advantages, the rotational forces to which the involute gear elements are subjected are also known to place substantial tensile stress or bending force on the root portions of the gear tooth.
Therefore, it is advantageous to strengthen the gear element to prevent failure from the various stresses applied to thereto. Strengthening typically takes the form of hardening by way of initial heat-treating of the entire gear element. Heat-treated gear elements are then commonly subjected to additional finishing steps applied to shape the surface asperity profile of the gear flanks in order to increase the overall micro-level hardness of the gear element. Two of the more common finishing steps are abrasive grinding and shot peening. With abrasive grinding, a grinding tool is used to mechanically shave or grind the entire profile of the gear tooth, including the exposed gear root portions. Complete shaving of the entire gear tooth profile is often considered necessary in order to avoid “steps” or unevenness along the continuous gear tooth surfaces. Common abrasive grinding techniques include diamond grinding and, more commonly, cubic born nitride or CBN grinding. With shot peening, also known as metal bead blasting, metal shot or small spheres are blasted or shot into the exposed gear surfaces to plastically deform the impacted surface layers to thereby introduce compressive residual stresses and increase the micro-hardness of the surfaces. However, because all of the exposed surfaces of the gear element are equally affected by the bombardment of metal shot, the asperity profile of the exposed flanks of the gear tooth may be altered beyond that which is desirable, and, as a result, the gear elements might have to be subjected to additional finishing steps such as polishing and/or glass-bead blasting.